Typically, many identical integrated circuits are constructed on a single wafer of semiconductor material, such as monocrystalline silicon or gallium arsenide. The portion of the wafer occupied by a single one of the integrated circuits is called a die. After completed fabrication of the integrated circuits, a series of tests (known as Probe) is performed in which the function of each die is tested. The test data collected for each die is used in subsequent assembly/packaging steps to ensure that only properly functioning die are packaged as integrated circuit chips.
Following the Probe test, the individual dies are separated from one another, and each properly functioning die is encapsulated (usually in plastic or ceramic) in a package having electric leads to form an integrated circuit chip. Subsequently, a series of testing operations is performed for each of the integrated circuit chips, with test data collected for each of the chips.
In order to properly correlate the various test results with the appropriate die or dies, accurate identification of each of the dies is required, both before and after packaging as an integrated circuit chip. The ability to identify particular dies can also be important following completed manufacture of the integrated circuit chip. For example, when an integrated circuit chip unexpectedly fails during later use, the manufacturer of the chip may wish to identify other potentially failing chips and the users of those chips.
Many of today's integrated circuits provide electronically readable identification information which is programmed into the integrated circuit itself—usually during Probe testing. Also, chip packages usually have an ink or laser-scribed mark which provides some useful information, such as date and country of manufacture, product and package types, speed and other test parameters, and manufacturing lot identification. However, information uniquely identifying the particular integrated circuit within a chip is included only in the electronically readable identification information.
The electronically readable identification information is usually available only during those manufacturing procedures in which the integrated circuit is electrically tested. Also, retrieving the electronically readable identification information is usually done with a relatively sophisticated semiconductor testing apparatus, which necessarily makes physical contact with the integrated circuit chip in order to access the identifying information. There exist a number of manufacturing process steps during which such physical contact does not occur, and hence traceable identification of the individual integrated circuit chips through the various manufacturing processes is quite difficult. Also, once the manufacturer ships the integrated circuit chip to a customer, the unique identification information is not readily available.